1. Field of the Invention
The present invention pertains to wireless communication networks. In particular, the present invention pertains to a wireless communication network with automatic control of time-of-day synchronization and merger with other networks.
2. Description of the Related Art
Due to the severe timing constraints imposed by tactical wireless ad-hoc networks, communication units or radios within that type of network must establish a common network time before they can start to communicate. This is accomplished by Time-Of-Day (TOD) synchronization. Conventional tactical radio systems, such as the NTDR (Near Term Data Radio) system, built by ITT, can operate with two different TOD algorithms. Selection of a TOD algorithm is based on a parameter in a pre-defined radio configuration. The primary TOD mode is global positioning system (GPS) based TOD, in which network TOD is slaved to GPS time. A secondary TOD mode, that is used when GPS is not available, is Brigade Time Head (BTH) based TOD.
In BTH based TOD synchronization, the network TOD is started by a single master (Brigade Time Head) radio. Time is then averaged across all participants in the network. If a radio network is started in BTH mode, the network remains operating solely in BTH mode. While operating within a BTH network, GPS configured radios ignore GPS signals. In the conventional NTDR TOD approach, no solution exists for converting a BTH network to a GPS based network once the respective TOD references differ by more than one epoch.
In GPS based TOD synchronization, at least one radio is deployed with local GPS and the network operates solely as a GPS based network (as long as GPS is available). If GPS signals are lost, the network converts to a BTH mode of operation. Once radios connect as a GPS based TOD network, the radios only exchange current net time with other radio nodes. The respective radios in the network stop looking for alternate sources of timing.
Furthermore, in a BTH based network, a radio can become operational only if it is in range of the network started by the BTH master. This means that radio nodes that are out of range of the network remain off-line until connectivity is provided back to the main network. The existing NTDR TOD protocol does not allow different time based networks to merge, even if they are geographically close and are within radio frequency (RF) range.
Recently introduced commercial wireless devices employ clock synchronization techniques for the operation of WPAN (Wireless Personal Area Networks) and WLAN (Wireless Local Area) Networks. For example, Bluetooth devices are WPAN type devices designed to support low bandwidth, short distance (<10 m) wireless connections. Bluetooth specifies the protocols to be used by different handheld computing devices in order to communicate and exchange data. For timing purposes, Bluetooth specifies a master/slave clock synchronization mechanism that allows synchronization between neighbors within one-hop from each other.
The IEEE 802.11 WLAN standard specifies two different approaches for time synchronization: (1) one approach for infrastructure-based networks and (2) one approach for independent networks. For infrastructure-based networks, IEEE 802.11 provides a master/slave clock synchronization mechanism. A special fixed node, the access point (AP), is used as a master, and all other nodes slave to this master. In an independent network, a mobile node transmits a beacon message, repeating on a beacon period. Each receiving node updates its clock with the value in the received beacon message if the received value is greater than its current local time. If the received value is less than the local time, the received value is discarded. Neither WPAN nor WLAN networks support merging of different TOD networks.
Tactical radios within the NTDR system, employ one receiver and one transmitter (1R/1T). Each radio tunes to a reservation channel whenever the radio is not involved in a message transaction. Since NTDR nodes can not monitor the reservation channel during a message transaction, timing messages can be lost. In new tactical radios, such as ITT's small unit operations (SUO) radio, this problem is corrected by adding an auxiliary receiver, thereby providing two receivers in the radio (2R/1T). When the transceiver of a SUO radio is used for a message transaction, the auxiliary receiver switches to monitor the reservation channel. In this manner, the reservation channel is monitored continuously. The extra receiver employed in the SUO system requires additional hardware/software, a capability not available in NTDR.
NTDR transmission security (TRANSEC) can operate from two different TOD sources. Selection of a TOD source is determined by a configurable parameter within the radio configuration. As previously described, normal TOD mode is GPS based TOD. BTH based TOD is optional and/or provides a fall back capability should GPS based TOD fail.
In GPS based TOD mode, network TOD is slaved to GPS time. A network member may receive GPS directly or receive a TOD message update from another radio (or node). Radios not receiving GPS signals are slaved to radios receiving GPS signals. To start a network, at least one of the radios in the network must receive a GPS signal. If the GPS signal is lost across the whole network for a predetermined period of time, the network members switch to a BTH based TOD mode. If a radio again begins to receive a GPS signal after the network has switched to BTH mode, the GPS signal is ignored and the radio remains operating in the BTH mode.
In BTH based TOD, the network TOD is generated from the average TOD received from all radios during a periodic, pre-defined interval. One radio in the network is pre-designated to be the Brigade Time Head Master by the radio configuration. This is the only radio that can bring up a network. Once the network is up, new members join the network by getting TOD through Cold Start (CS) and Late Net Entry (LNE) messages. Since the BTH master station is only used to start the network, if the master station disappears after the network is started, the BTH network is not adversely affected. Splintered networks can rejoin only if their respective network TODs remain within 80% of a common TOD epoch.
In conventional TOD protocols, only nodes without a selected timing source search for CS and LNE messages. Once these messages are received, the node proceeds to make a timing source selection from the information in these messages. When a node is an operational member of a network, or In-Net (i.e., is able to send data messages via a network data channel to other member radio nodes of a network), the node may periodically transmit CS and LNE messages to bring in other nodes that are unaffiliated. Nodes that are operating in the In-Net state may not listen for other CS or LNE messages. Therefore, these nodes are typically unaware that other network(s) may exist operating with a time offset from their network.
The TOD synchronization scheme of the NTDR system has been modified to meet small unit operations (SUO) requirements for radio operation in extremely mobile, volatile environments under power and bandwidth limited conditions. The basic functionality of CS, LNE and In-Net modes of the NTDR system are similar to Isolated, In-Sync, and Associated modes employed in the SUO system (2R/1T and 2R/2T). The SUO TOD system, however, includes a “flywheel” synchronization scheme by which a roaming radio node with local GPS can synchronize with the existing (non-GPS based) net time, immediately, and slowly pull the net time toward the GPS based time.
An exemplary NTDR wireless network is illustrated in FIG. 1A. Specifically, wireless network 2 includes a plurality of nodes 10 arranged in cells or clusters 12 in accordance with predetermined clustering rules. Each cell or cluster includes corresponding cluster member (CM) nodes 10 with one of those cluster member nodes designated as a cluster head (CH) node 14. These cluster arrangements form a first tier of network 2. Communication within a cluster is facilitated by the cluster head which maintains information related to the cluster. For example, in the case of a small isolated network with a single cluster head, the cluster head may maintain an information store that contains information specific to its respective cluster (i.e., its isolated network), such as the total number of nodes in the network, the number of active nodes in the network, traffic loads through portions of the network (i.e., between identified cluster members), etc.
In accordance with predetermined clustering rules, cluster heads typically establish communication with each other, preferably utilizing a second transmission frequency, and form a backbone network 18. The backbone network essentially forms a second tier of network 2 and facilitates communications between nodes of different clusters (e.g., generally providing communications over greater distances). The architecture of network 2 is similar to that of conventional cellular telephone systems, except that network 2 provides dynamic selection of cells and cluster head nodes.
A typical network node 10 is illustrated in FIG. 1B. Specifically, node 10 includes a transmitter 22, a receiver 24 and a processor 26. The processor is preferably implemented by a conventional microprocessor or controller and controls the node to transmit and receive messages in accordance with communication protocols. The transmitter is preferably implemented by a conventional transmitter and transmits messages from the processor, preferably in the form of radio frequency (RF) signals, in accordance with processor instructions. Receiver 24 is typically implemented by a conventional receiver and configured to receive signals, preferably in the form of radio frequency (RF) signals, transmitted by the transmitter of another node. The receiver receives transmitted signals and forwards the received signals to processor 26 for processing. The node further includes an identifier (e.g., a code or identification number) to identify the particular node and a database (not shown) to store information pertaining to neighboring nodes. Cluster head (CH) nodes 14 are substantially similar to node 10 described above.
Prior to establishment of the network described above with respect to FIG. 1A, individual nodes are started (i.e., powered up) and the network is initiated based upon predetermined rules defined within each respective node. FIGS. 2A and 2B present conventional NTDR node startup process flows by which an individual node powers up and initiates a new network or joins an existing network. Startup of an NTDR node configured to join a network using GPS based TOD synchronization is presented in FIG. 2A. Startup of an NTDR node configured to join a network using BTH based TOD synchronization is presented in FIG. 2B.
Referring to FIG. 2A, upon start of a radio node configured to join a GPS TOD based network, at step 102, the radio node enters a cold start (CS) mode, at step 104, sets node status to Isolated (i.e., sets an isolated flag to “ON”), at step 106, and waits for receipt of a cold start (CS) packet, at step 108. Cold start (CS) messages allow Isolated nodes to join the network. Isolated nodes can be offset in TOD by large deltas from the In-Net TOD.
When an Isolated node receives any CS message, as determined at step 110, the local clock is updated with the neighbor's TOD, at step 112, and the node sets its status to In-Sync, at step 114. It is important to note that a status of In-Sync merely indicates that the isolated radio node has successfully synchronized its local clock to the TOD contained within a CS message received from a neighbor node. The isolated node remains isolated (i.e., is unable to send data messages via a network data channel to another radio node) until the isolated node achieves in network, or In-Net, status as described below.
The Isolated node waits to receive additional LNE messages to further correct its time, at step 116, before it changes the status of the radio node to In-Net, at step 118. If no further LNE messages are received, at step 116, within a predetermined period of time, the node returns to cold start mode and processing resumes at step 104, as described above.
If, at step 110, no CS message is received, yet the node determines that it has access to a local GPS signal, at step 120, the node declares its status as In-Net, at step 122, and starts to send CS, LNE and TOD messages, at step 124, until a predetermined period of time (e.g., WaitInNetTimer) expires, at step 126. The CS, LNE and TOD messages sent out by the node at step 124, are sent out at an accelerated rate (i.e., based upon an interval timer TODinterval of short duration), so that many nodes can join the network during the initial TOD operation.
Regardless of the startup path taken by the node to achieve In-Net status, as described above, once the startup process is complete, the node proceeds to periodically send/receive, at step 128, CS, LNE and In-Net TOD messages in order to distribute TOD information to neighboring nodes, establish contact with unaffiliated nodes, and to receive TOD updates from neighboring nodes. If upon receiving a message the receiving radio node determines that GPS TOD time is available, at step 130, the receiving node ignores, at step 132, the TOD update. If, upon receiving a message, the receiving radio node determines that GPS TOD time is not available, the receiving node accepts, at step 134, the TOD update. Next, at step 136, the receiving node processes the TOD update to determine whether to use the TOD contained therein. In one embodiment, a receiving node is configured to update its TOD to the TOD of a neighboring node with the fewest number of links to the GPS signal TOD source. In case of a tie with respect to number of hops, the receiving node is configured to choose the TOD of the neighboring node with the fewest number of links to the GPS signal and the lowest node identifier. The node continues to send and receive CS, LNE and In-Net TOD messages in such a manner so long as contact with neighboring nodes is maintained. Upon determining, at step 138, that no messages have been received in a predetermined amount of time, the node returns to cold start (CS) state, and processing continues at step 104.
Startup of an NTDR node configured to join a network using BTH based TOD synchronization is presented in FIG. 2B. As shown in FIG. 2B, upon start of an NTDR radio node configured to join a BTH based network, at step 202, the radio node sets node status to Isolated (i.e., sets an Isolated flag to “ON”), at step 204, and waits, at step 206, to receive an LNE message.
Upon receipt, at step 206, of an LNE message, the radio node sets, at step 208, its real time clock (RTC) to the received TOD, sets, at step 210, node status to In-Sync (i.e., sets an In-Sync flag to “ON”) and sets, at step 212, an In-Sync wait timer. If the radio node receives, at step 214, an In-Sync message from another network node before expiration, at step 216, of the In-Sync timer, the radio node sets, at step 218, node status to In-Net (i.e., sets an In-Net flag to “ON”) and proceeds, at step 220, with execution of In-Net TOD processing as described below with respect to FIG. 3.
If no LNE message is received, at step 206, and the radio node determines, at step 222, that the radio node is a BTH master clock node, the radio node sends, at step 224, a series of brigade (BDE) beacon messages (e.g., one beacon message per BeaconInterval of 1 second for a predetermined period of time) to inform other radios that it is the master radio. The other non-BTH-master radios remain silent until they receive a BDE beacon message. Upon receiving a BDE beacon message, each radio sends out a BDE beacon acknowledgement (ACK) message indicating that it is listening. The BTH master node receives, at step 226, the beacon ACK messages and sets, at step 228, a timer of duration WaitBeaconAckTime (e.g., 4 minutes) to allow time, at step 230, for other radios to turn on. Upon expiration, at step 230, of the WaitBeaconAckTime timer, the master radio, at step 232, sets status to In-Sync (i.e., sets an In-Sync flag to “ON”) and sends out, at step 234, multiple LNE and In-Sync messages. For example, in one embodiment, the BTH master sends one LNE message and one In-Sync TOD message every InSyncXmtInterval (e.g., 0.5 seconds) for a duration of InSyncXmtDuration (e.g., sixteen seconds). After expiration, at step 236, of the InSyncXmtDuration interval, the master radio switches, at step 218, its node status to In-Net (i.e., sets an In-Net flag to “ON”) and proceeds, at step 220, with execution of In-Net TOD processing as described below with respect to FIG. 3. Until a radio is in In-Net operation, no data traffic is passed on a network except TOD update messages. It is important to note that LNE messages are transmitted upon the reservation channel and In-Sync TOD messages are transmitted over the data communication channel. In this manner, while in In-Sync mode, a radio node continues to receive TOD updates, even when the reservation channel is not being monitored.
FIG. 3 presents an example of TOD maintenance and In-Net status processing with respect to an NTDR node within a BTH network, initiated as described with respect to FIG. 2B. As shown in FIG. 3, upon a radio node achieving In-Net status, at step 302, the node broadcasts, at step 304, LNE TOD messages at a rate determined by the cluster head or cluster member status of the node. For example, the delay between broadcasts is typically a fixed delay supplemented with a random delay. In a cluster head node, the fixed delay is typically two minutes and the random delay is between 0 and 2 minutes. In a cluster member node, the fixed delay is typically eight minutes and the random delay is between 0 and 2 minutes.
Upon sending, at step 304, LNE TOD messages, as described above, the node sets, at step 306 an AverageNetTime (e.g., 2 minutes) timer and proceeds to compute, at step 308 the average delta time associated with TODs received from neighboring nodes prior to expiration, at step 310, of the AverageNetTime timer. The average delta time is a mathematical average of the TOD values received in LNE TOD messages received during the AverageNetTime period (e.g., 1 second). Use of an average delta time calculated from TOD information received from neighboring nodes during a fixed time interval, in such a manner, prevents a sudden change in network time. Upon expiration of the AverageNetTime timer, at step 310, the radio node updates, at step 312, its TOD with the computed average delta time, calculated in step 308, and proceeds to conduct normal communications, at step 314, using the updated TOD. If the radio node receives any messages from neighboring nodes prior to expiration, at step 316, of a CheckInNetThresold (e.g., twenty minutes) timer, the node remains in In-Net mode and TOD processing continues at step 302, otherwise, if the node fails to receive any messages from neighboring nodes prior to expiration, at step 316, of a CheckInNetThresold timer, the node's status changes, at step 318, to Isolated and the radio node again tries to join the existing net in accordance with the process described above with respect to FIG. 2B.
During operational use of NTDR radios, a number of drawbacks have been encountered that are due, to a large extent, upon the conventional NTDR TOD algorithms described above, with respect to FIGS. 1A-3. For example:                Radios deployed and configured for GPS based TOD require at least one radio to acquire GPS to start the network. Failure of at least one radio to acquire GPS prevents the deployed network from forming.        Radios deployed and configured for BTH based TOD, require the master radio to start the network. Failure of the master radio to turn-on or to be available to the current network prevents the deployed network from forming.        Radios not in direct contact with the BTH master radio or a radio with GPS access may fail to startup, thereby leaving off a large section of the network.        When a BTH network fragments for longer than a maximum network separation time, the network may permanently fragment and have no way to re-merge.        There exists no merge mechanism in the existing approaches to combine a BTH based network with other BTH network(s) or GPS based network(s).        There exists no mechanism in the existing approaches to move a network operating in BTH based TOD network to a GPS based TOD network.        LNE epoch periods are quantized to six-minute increments because the TOD stamped at the link layer header is not the actual TOD time when the over-the-air message is sent. This large LNE epoch period causes slower TOD synchronization.        
Hence, a need remains for a method and apparatus for synchronizing TOD and for merging isolated devices/separate networks within a wireless communication environment. Preferably, such an approach would allow individual radios to start up and establish a communication network with surrounding radio devices without connectivity or access to an existing network node, a master radio and/or a GPS signal. Further, such an approach would allow isolated devices and/or separate networks to merge regardless of whether the respective devices and/or networks are operating in GPS or BTH TOD types of synchronization modes.